The present invention relates to magnetic coils for producing highly uniform magnetic fields, such as those required for magnetic resonance imaging, and particularly to a method and apparatus for compensating for magnetic field drift of a superconducting coil.
As is well known, a magnet coil wound of superconductive material can be made superconducting by placing it in an extremely cold environment. For example, a coil may be made superconducting by enclosing it in a cryostat or pressure vessel containing a cryogen. The extreme cold enables the superconducting wires to be operated in the superconducting state. In this state, the resistance of the wires is practically zero. To introduce a current flow through the coils, a power source is initially connected to the coils for a short time period. In the superconducting state, the current will continue to flow through the coils, thereby maintaining a strong magnetic field. In other words, because superconductive windings offer no resistance to electrical current flow at low temperatures, the superconducting magnet is persistent. The electric current that flows through the magnet is maintained within the magnet and does not decay noticeably with time. Superconducting magnets find wide application such as in the field of magnetic resonance imaging (“MRI”), and most of these system have the active shielded feature.
Unlike conventional magnets, an actively shielded magnet is unable to automatically compensate (i.e., via Lens's Law) for the magnetic disturbances to the B0 field in the imaging volume due to external magnetic sources. This so because of the actively shielded magnet's combination of positive and negative turns. Thus, the actively shielded magnet only partly compensates for the shift in the B0 field. A B0 coil is a secondary coil added to an actively shielded superconducting magnet to shield the effects of moving metal objects in the vicinity of the magnet. B0 coils typically have a small mutual inductance with the primary coil
If the static magnetic field is significantly inhomogeneous, undesirable artifacts will occur in the image data. The uniform magnetic field is developed by a main magnetic coil and several active correction coils which are disposed on a cylindrical surface. The magnetic field produced by the coils is oriented in an axial direction with respect to the hollow cylinder on which the coils are disposed. The main magnetic coil is designed to produce as uniform a field as is practical. However, even when extraordinary steps are taken to ensure proper construction of the main coil and magnet field uniformity, some spatial field uniformity errors remain. Accordingly, it is conventional practice to employ relatively low power active correction coils to perturb the static magnetic field from the main coil in a manner which increases the overall field homogeneity.
Once the highly homogeneous magnetic field has been so established, the superconducting coils are maintained in the superconducting state for months at a time. However, all superconducting coils have a small but finite resistance and as a result, the coil currents decay slowly over time. This decay causes a drift in the static magnetic field within the cylindrical volume. The field drift due to the main coils' current decay will also induce additional currents in the magnetic coupled correction coils which produces a change in their magnetic flux contributions. The alteration of the magnetic flux produced by the additional current induced in the correction coils changes their contribution to the correction of the magnetic field from the main coil. Consequently, over a long period, the drift induced by main coils will degrade the homogeneity of the B0 magnetic field within the cylinder. As a result, a service technician must periodically go through the laborious and expensive process of measuring the field throughout the cylinder and re-adjusting these electrical currents of main coils and correction coils to homogenize the B0 field.
The magnetic field drift level, particular for a MRI system made with superconductivity material such as NbTi wires, will depend on the wire quality, superconducting joints, operation temperature, as well as the magnetic field level. The typical magnetic field drift rate for low temperature superconducting magnet system used for MRI purpose ranges from a few PPM (parts per million) to a few thousands PPM. As discussed above, the magnetic field is always drifting, as such, the question then becomes how much and its effect on imaging quality.
Therefore, it is desirable to further compensate for the magnetic field drift to either prolong the period between service points or to complete eliminate the current re-adjustment process.